The present invention relates to a rotary member and its production process. For examples, the rotary member may be: a grease-sealed bearing used in alternator of motor vehicle, electromagnetic clutch, idler pulley and other accessory equipment for an engine; a rolling bearing used in fuel injector pump; a rolling bearing for supporting a torque transmitting member such as a gear in a multi speed transmission, and a rolling member in a continuously variable transmission; a rolling bearing used in a compressor of air conditioner; a rolling member such as discs and power rollers in continuously variable transmission; and a gear of any of various types used in motor vehicle. More specifically, the present invention relates to the structure and production process of a rotary member adapted to restrain short-life flaking like hydrogen embrittlement due to hydrogen penetrating into base material such as steel in the rotary member.
In recent years, there are demands for power trains of smaller size and lighter weight, to improve fuel efficiency and increase power. Accordingly, rotary members such as rolling bearings, gears, and the like used in various portions in a power train are required to bear severe conditons of heavy load and high rotational speed with the construction of smaller size and lighter weight.
In the case of a grease-sealed bearing for alternator or electromagnetic clutch, the bearing is operated in stringent environment of heavy load, high speed and strong vibration due to vibration of engine. As a result, the use for only short time could cause structure change of peculiar mode underneath the rolling surface, and the resulting flaking as reported in (D1) “A new Type of Microstructural Change in Bearings for Electrical Instruments and Auxiliary Devices of Automotive Engines” NTN Technical Review No. 61 (1992), (D1′) “Brittle Flaking on Bearings for Electrical Instruments and Auxiliary Devices and Life Extension of these Bearings” NTN Technical Review No. 61 (1992), and (D2) “Study on Fatigue Mechanism of Bearings for Automotive Alternators”, NSK Technical Journal No. 656 (1993).
The cause for this short-life flaking is thought to be that the use in the severe environment of heavy load, high speed rotation, and strong vibration causes mirror surface wearing in rolling contact surface, the formation of fresh metal surface caused by this wear plays a catalytic role to decompose grease, and the hydrogen thus generated intrudes into steel, which results in the hydrogen embrittlement like flaking.
In the case of a rolling bearing in a compressor of an air conditioner, the bearing is lubricated with a mixture of lubricant and refrigerant. For the protection of earth environment, a new refrigerant of a type including hydrofluorocarbon (HFC) is used as alternative to flon. With this replacement of the refrigerant, the lubricant is replaced, too, from the lubricant prepared from petroleum such as naphthene-type and paraffin-type, to lubricant soluble in HFC such as polyalkylene glycol (PAG) or polyolester (POE). As a result, a peculiar structure change could occur in the inside under the rolling contact surface and cause flaking before long, as recited in (D3) Tribologist, Vol. 37, No. 11 (1992), and (D4) Published Japanese Patent Application Kokal No. H08(1996)-177864.
This short-life flaking is probably caused by microsopic metal contact generated between a rolling element and a bearing ring under the new condition of film of the new refrigerant. Fresh metal surface bared by this metal contact acts to decompose hydrocarbon or water in the lubricant, and the resulting hydrogen intrudes into the metal and enbrittles the internal structure.
As measures against the above-described short-life hydrogen embrittlement like flaking, there have been proposed technique in which triiron tetroxide is formed on the rolling surface by blackening treartment or black oxide finishing (as proposed in (D5) Published Japanese Patent Application Kokai No. H02(1990)-190615≈U.S. Pat. No. 5,150,974); technique in which a lubricant containing a deactivator is used to reform the rolling surface so that the reaction of deactivator is accelerated (as proposed in (D6) Published Japanese Patent Application Kokai No. 2001-20958); and technique in which the content of Cr of base material is increased to form an inert oxide film such as FeCrO4 on the surface (as proposed in (D4) Published Japanese Patent Application Kokai No. H08(1996)-177864).